296 THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 



If any advance upon the results of former investigators of selachian-nerve histogenesis 

 has been made in this research, this may be attributed chiefly to the application of 

 somewhat improved neurological methods to very early stages of nerve histogenesis 

 and to a more extended study of selachian nerves in all stages of development than 

 has been previously made. The work was done chiefly at the Marine Biological 

 Laboratory at Wood's Hole during the summers of 1898 and 1900. To the directors of 

 the laboratory, Dr. C. 0. Whitman and Dr. F. R. Lillie, my sincere thanks for the free 

 use of an investigator's room and for many unusual privileges are here gladly given. 



II. METHODS. 



The main results of the present investigation are based on the study of Squalus 

 acanthias (spiny dogfish) embryos treated after the vom Rath ('93, '95) method. 

 For purposes of comparison Hermann's, Flemming's, and Davidoff's fluids, followed 

 by anilin safranin or iron hsematoxylin, were used. With the exception of a few 

 embryos in advanced stages of development, the Golgi rapid method, which was 

 tried upon over a hundred embryos, failed to give any results. 



The vom Rath method, which has been used with considerable success in the 

 study of the nervous system of annelids, but never, so far as I know, recommended 

 for the study of nerve histogensis in vertebrates except by the present writer ('98), 

 is as follows : 



1. The embryos are killed and fixed in either of the following mixtures, in which 

 they remain at least twenty-four hours: 



A. vom Rath's ('93) formula: 



a. Saturated and filtered solution of picric acid 500 c.c. 



b. Glacial acetic acid 3 c.c. 



c. Platinic chloride (dissolved in 5. c.c. water) 5 gms. 



d. Osmic acid 2 gms. 



B. vom Rath's ('95) formula: 



a. Saturated solution of picric acid 200 c.c. 



b. Glacial acetic acid 2 c.c. 



c. Platinic chloride (dissolved in 10 c.c. water) 1 gm. 



d. Osmic acid, 2% 25 c.c. 



Larger embryos, 30-50 millimetres, are better preserved by leaving them in 

 the fixing fluid — which is changed for fresh every day — for three to four days. In 

 making these changes of fluid and subsequent changes it is necessary to avoid shaking 



